As is known in the art, magnetic sensing devices which can detect the presence of a ferromagnetic object in the vicinity of the sensing device have been widely used. Such sensing devices typically utilize a magnetic field and employ sensing apparatus that detect changes in the strength of the magnetic field. Magnetic field strength is defined as the magnetomotive force developed by a permanent magnet per the distance in the magnetization direction. As an example, an increase in the strength of a magnetic field, corresponding to a drop in the reluctance of a magnetic circuit, will occur as an object made from a high magnetic permeability material, such as iron, is moved toward the magnet. Magnetic permeability is the ease with which the magnetic lines of force, designated as magnetic flux, can pass through a substance magnetized with a given magnetizing force. Quantitatively, it is expressed as the ratio between the magnetic flux density (the number or lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force. Because the output signal of a magnetic field sensing device is dependent upon the strength of the magnetic field, it is effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in Gauss or teslas.
As is also known, where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object is typically provided with surface features that project toward the sensing device, such as teeth. The proximity of a tooth to the sensing device will increase the strength of the magnetic field. Accordingly, by monitoring the output of the sensing device, the rotational speed of the disk can be determined by correlating the peaks in the sensor's output with the known number of teeth on the circumference of the disk. Likewise, when the teeth are irregularly spaced in a predetermined pattern, the rotational position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
One prominent form of such a sensing device is a Hall effect transducer. A Hall effect transducer relies upon a transverse current flow that occurs in the presence of a magnetic field. The Hall effect transducer is primarily driven by a direct current (DC) voltage source having electrodes at both ends of the Hall effect transducer, creating a longitudinal current flow through the sensor's body. In the presence of a magnetic field, a transverse voltage is induced in the transducer that can be detected by a second pair of electrodes transverse to the first pair. The second pair of electrodes can then be connected to a voltmeter to determine the potential created across the surface of the sensor. This transverse voltage increases with a corresponding increase in the magnetic field's strength.
The Hall effect transducer can also be chopper stabilized, wherein for half of some clock cycle the first pair of electrodes receives the DC voltage source creating the longitudinal current flow through the sensor's body. For the second half of the clock cycle, the second pair of electrodes receives this energizing DC voltage. This well-known technique reduces the inherent electrical offsets that may be present in the Hall effect transducer.
The Hall effect transducer is most often integrated into a single integrated circuit that contains conditioning circuitry to amplify and otherwise modify the output of the Hall effect transducer. This integrated circuitry is often referred to as a Hall effect sensor.
The Hall effect sensor is mounted within and perpendicular to a magnetic circuit that can include a permanent magnet and an exciter (the object being sensed). The exciter is a high magnetic permeability element having projecting surface features which increase the strength of the magnet's magnetic field as the distance between the surface of the exciter and the permanent magnet is reduced. Typically, the exciter will be in the form of a series of spaced teeth separated by slots, such as the teeth on a gear. The exciter moves relative to the stationary Hall effect sensor element, and in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the Hall effect element to vary in a manner corresponding to the position of the teeth. With the change in magnet flux there occurs the corresponding change in magnetic field strength, which increases the transverse voltage of the Hall effect sensor.
The Hall effect sensor can also detect the proximity of a permanent magnetic material, such as a rotating ring magnet.
With the increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics in their operation, such as automobile control systems. Common examples of automotive applications are the detection of ignition timing from the engine crankshaft and/or camshaft, and the detection of wheel speed for anti-lock braking systems and four-wheel steering systems. For detecting wheel speed, the exciter is typically an exciter wheel mounted inboard from the vehicle's wheel, the exciter wheel being mechanically connected to the wheel so as to rotate with the wheel. The exciter wheel is provided with a number of teeth which typically extend axially from the perimeter of the exciter wheel to an inboard-mounted magnetic field sensor. As noted before, the exciter wheel is formed of a high magnetic permeability material, such as iron, such that as each tooth rotates toward the sensor device, the strength of the magnetic field increases as a result of a decrease in the magnetic circuit's reluctance. Subsequently, the magnetic circuit reluctance increases and the strength of the magnetic field decreases as the tooth moves away from the sensing device. In the situation where a Hall effect device is used, there will be a corresponding peak in the device's potential across the transverse electrodes as each tooth passes near the device.
The sensor's output is dependent upon the distance between the exciter and the sensing device, known as the air gap. More specifically, as the air gap increases, the maximum output range of the device decreases thus decreasing the resolution of the output and making it more difficult to accurately analyze the device's output. The output of a Hall effect device is directly proportional to the strength of the magnetic field, and therefore is sensitive to the air gap at low strength magnetic fields.
An integrated circuit magnetic field sensor assembly typically can include a magnet, and one or more of the above-described Hall effect semiconductor sensors, where the magnetic poles are sensed as they move relative to the Hall effect semiconductor sensor. The assembly can also include a permanent magnet mounted proximal to the Hall effect semiconductor sensor, and a ferrous object then moves relative to the Hall effect semiconductor sensor, and this sensor detects the disturbances in the magnetic field created by the passing ferrous object. The magnet provides a magnetic field. The semiconductor sensors are located within the magnetic field and are utilized for sensing the strength of the magnetic field. The magnetic field sensor allows the detection of a ferromagnetic object passing through the magnetic field. The magnetic field sensor is disposed adjacent to the ferromagnetic object and is positioned from the ferromagnetic object so as to reduce the distance between the magnetic field sensor and the ferromagnetic object and still maintain an air gap between the magnetic field sensor and the ferromagnetic object to allow passage of the ferromagnetic object by the magnetic field sensor. The semiconductor sensors may be realized as Hall effect devices which are used to detect edges of the ferromagnetic object such as a gear tooth. From the detection of the edges of a gear tooth, information relating to the speed and direction of the rotating gear can be determined.
While detecting the speed of and direction of rotating devices has proven useful, further information regarding the rotating device is generally not available. It would, therefore, be desirable to provide a method and apparatus which provides information relating to the speed and direction of a rotating device and to further provide information relating to the environment the device is disposed in.